System and method for measuring conductivity of fluid

Abstract

A system and related method are provided to calibrate for wire capacitance during use to minimize error in conductivity measurement of the target fluid. The system includes a signal generator configured to drive the conductivity cell and the temperature element, with an alternating current (AC) drive signal having variable parameter. The system further includes a processor assembly electrically coupled to the conductivity cell and the temperature element to calculate a conductivity value of the fluid. The conductivity value is a function of the values of the temperature measurement, the compensation measurement, and the raw conductivity measurement, thereby compensating the conductivity value for capacitance effects. In this manner, the system effectively compensates for capacitance attributable to wiring extending between the electrode and other electronics of the sensor, usable with wiring of varied and unknown lengths.

1. A system for measuring conductivity of a fluid, comprising:
 a signal generator for driving a conductivity cell configured to contact a target fluid and a temperature element configured to measure the temperature of the target fluid, the signal generator configured to generate an alternating current (AC) drive signal having variable parameters, the signal generator configured to (a) drive the temperature element with a first drive signal having an initial frequency that is greater than or equal to zero, resulting in a temperature measurement, (b) drive the temperature element with a second drive signal having an initial frequency that is greater than the initial frequency of the first drive signal, resulting in a compensation measurement, and (c) drive the conductivity cell with an AC drive signal having a third frequency greater than the first frequency, resulting in a raw conductivity measurement; and 
 a processor assembly electrically coupled to the conductivity cell and the temperature element, the processor assembly configured to calculate a conductivity value of the fluid that is compensated by the conductivity value for capacitance effects, such that the compensated conductivity value is a function of the values of the temperature measurement, the compensation measurement, and the raw conductivity measurement. 
 
   
   
     2. A system as defined in  claim 1 , wherein the processor assembly compensates for capacitance effects attributable to wiring that connects components of the conductivity cell and the temperature element to components of the processor. 
   
   
     3. A system as defined in  claim 1 , wherein the signal generator is configured to produce an AC drive signal for driving the conductivity cell that is proportionally related to the value of the raw conductivity measurement. 
   
   
     4. A system as defined in  claim 1 , wherein the signal generator includes a first capacitor having a capacitance value proportionally related to the numerical value of the frequency of the second drive signal for the temperature element and to the numerical value of the frequency of the third AC drive signal for driving the conductivity cell, and a second capacitor having a capacitance value proportionally related to the numerical value of the frequency of the first drive signal for the temperature element. 
   
   
     5. A system as defined in  claim 1 , wherein the signal generator is configured to provide a first drive signal having a frequency that is at least ten times less than the frequency of the second drive signal. 
   
   
     6. A system as defined in  claim 1 , wherein the signal generator includes an integrator op-amp and a comparator op-amp, the integrator op-amp coupled to a first capacitor and a second capacitor having different capacitance values. 
   
   
     7. A system as defined in  claim 6 , wherein the signal generator includes a first capacitor having a capacitance value proportionally related to the numerical value of the frequency of the second drive signal for the temperature element and to the numerical value of the frequency of the third AC drive signal for driving the conductivity cell, and a second capacitor having a capacitance value proportionally related to the numerical value of the frequency of the first drive signal for the temperature element. 
   
   
     8. A system as defined in  claim 1 , further comprising a conductivity cell, a temperature element, a sensor housing that accommodates the conductivity cell and the temperature element, and an electronics housing that accommodates selected components of the signal generator and selected components of the processor assembly. 
   
   
     9. A method for measuring conductivity of a fluid, comprising:
 driving a temperature element exposed to a target fluid with a first drive signal having an initial frequency that is greater than or equal to zero, resulting in a temperature measurement; 
 driving the temperature element with a second drive signal having an initial frequency that is greater than the initial frequency of the first drive signal, resulting in a compensation measurement; 
 driving a conductivity cell exposed to a target fluid with an AC drive signal having a third frequency greater than the first frequency, resulting in a raw conductivity measurement; and 
 calculating a conductivity value of the fluid that is selectively compensated for capacitance effects, wherein the conductivity value is a function of the values of the temperature measurement, the compensation measurement, and the raw conductivity measurement. 
 
   
   
     10. A method as defined in  claim 9 , wherein generated parameters of the AC drive signal for the conductivity cell are proportionally related to the conductivity measurement signal. 
   
   
     11. A method as defined in  claim 9 , wherein the signal generator is configured to provide a first drive signal having a frequency that is at least ten times less than the frequency of the second drive signal. 
   
   
     12. A method as defined in  claim 9 , wherein the signal generator includes an integrator op-amp and a comparator op-amp, the integrator op-amp coupled to a first capacitor and a second capacitor having different capacitance values. 
   
   
     13. A method as defined in  claim 12 , wherein the signal generator includes a first capacitor having a capacitance value proportionally related to the numerical value of the frequency of the second drive signal for the temperature element and to the numerical value of the frequency of the third AC drive signal for driving the conductivity cell, and a second capacitor having a capacitance value proportionally related to the numerical value of the frequency of the first drive signal for the temperature element. 
   
   
     14. A circuit for measuring conductivity of a fluid, comprising:
 means for driving a conductivity cell configured to contact a target fluid and a temperature element configured to measure the temperature of the target fluid, the signal generator configured to generate an alternating current (AC) drive signal having variable parameters, the signal generator configured to (a) drive the temperature element with a first drive signal having an initial frequency that is greater than or equal to zero, resulting in a temperature measurement, (b) drive the temperature element with a second drive signal having an initial frequency that is greater than the initial frequency of the first drive signal, resulting in a compensation measurement, and (c) drive the conductivity cell with an AC drive signal having a third frequency greater than the first frequency, resulting in a conductivity measurement; and 
 means for calculating a conductivity value of the fluid that is a function of the values of the temperature measurement, the compensation measurement, and the raw conductivity measurement, thereby compensating the conductivity value for capacitance effects. 
 
   
   
     15. A circuit as defined in  claim 14 , wherein the means for driving includes a first capacitor having a capacitance value proportionally related to the numerical value of the frequency of the second drive signal for the temperature element and to the numerical value of the frequency of the third AC drive signal for driving the conductivity cell, and a second capacitor having a capacitance value proportionally related to the numerical value of the frequency of the first drive signal for the temperature element. 
   
   
     16. A circuit as defined in  claim 14 , wherein the means for driving is configured to produce an AC drive signal for driving the conductivity cell that is proportionally related to the value of the raw conductivity measurement. 
   
   
     17. A circuit as defined in  claim 14 , wherein the means for driving includes an integrator op-amp and a comparator op-amp, the integrator op-amp coupled to a first capacitor and a second capacitor having different capacitance values. 
   
   
     18. A circuit as defined in  claim 17 , wherein the first capacitor having a capacitance value proportionally related to the numerical value of the frequency of the second drive signal for the temperature element and to the numerical value of the frequency of the third AC drive signal for driving the conductivity cell, and a second capacitor having a capacitance value proportionally related to the numerical value of the frequency of the first drive signal for the temperature element.